As is known in the art, in semiconductor fabrication critical dimensions below 1 micron are typically measured with top-down scanning electron microscopy. Although a useful measurement tool, scanning electron microscopes have several disadvantages. These disadvantages include, among other things, charging a sample to be measured and increased risk of contamination.
Scanning electron microscopy (SEM) is used principally to provide images at or near the surface of a solid, such as a semiconductor chip. By scanning an electron beam across the surface of a specimen, a one-to-one correspondence can be set up in the positions of the probe upon the specimen and the electron beam on an imaging device, for example an oscillograph. The signal produced is brightness modulated to provide a visual image. As described, SEM requires the bombardment of the specimen with an electron beam. This charges the specimen and may cause damage to semiconductor devices. Further, the electron beam may produce mass transport of atoms on the specimen which may lead to contamination of various regions of the semiconductor device. SEM and atomic force microscopy (AFM) are also limited, in that these techniques primarily measure individual structures on the surface of a semiconductor device rather than structure assemblies.
Therefore, a need exists for a method and system with improved resolution for subquarter micron measurements. A further need exists for a method and system for making such measurements without charging on contaminating a sample being measured. A still further need exists for performing such measurements on structure assemblies thus improving the statistical value of the measurement. Further, there is a need to measure taper angles (i.e., vertical profiles of features) in addition to critical dimensions (i.e., lateral dimensions) of submicron structures.
As we have described in our issued U.S. Pat. No. 6,031,614, the entire subject matter thereof being incorporated herein by reference, a method and apparatus for determining critical dimensions. Such method and apparatus include a conventional microellipsometer provided with a revolving stage. The method and apparatus realize a new kind of measurement tool which we call an Anisotropy Micro-Ellipsometer (AME) (i.e., a revolving ellipsometer). As explained in our U.S. Pat. No. 6,031,614, such an AME allows the measure of critical dimensions (cd) of sub-micron structure assemblies with high resolution without any of the aforementioned drawbacks. Conventional ellipsometry is well known in the art, and offers the advantage of being non-destructive and non-invasive to a sample. Conventional ellipsometers are used to measure optical parameters of surfaces and thickness of films which cover surfaces. For this, measurements on stationary samples are performed. Here, we extend the methods described in our U.S. Pat. No. 6,031,614 to measure vertical dimensions without requiring top-down and cross-sectional SEMs, i.e., the method in accordance with this invention is non-destructive. Further, film thickness can be monitored simultaneously. The method provides sufficient precision for groundrules of 100 nm and below.
Thus, in summary, the production of sub-micron microelectronic devices requires an accurate measurement of both the lateral (usually called “critical dimensions”, cd's) as well as vertical sizes (e.g., thickness, depth) of the design structures. In the general case of a vertical profile, an additional quantity is necessary for a complete description of the structure, such as the taper angle, τ. The taper angle quantifies the variation of critical dimension as a function of structure depth. This invention uses the optical methods which we refer to as Spectroscopic Anisotropy Micro-Ellipsometry (SAME) to determine all three quantities, i.e., critical dimension, thickness and taper angle of patterned structures, such as deep trench DRAM arrays or contact holes, non-destructively. For this, the invention makes use of:                (1) wavelength dependence of the penetration depth, δ, of a light beam which is reflected from a sample surface, where δ=λ/(2nk); with λ being the wavelength of the light and k being the absorption coefficient which itself is dependant on λ as well;        (2) combination of the well-known Bruggeman effective medium theory (BEMA) with SAME. The BEMA approach assumes mixtures or structures on a scale smaller than the wavelength of light, but that each constituent retains its original response and can therefore be applied to sub-micron structures. The generalized form for BEMA is:                     ∑        f            ⁢                        f          i                ⁢                                            N              i                        -                          N              ave                                                          N              i                        +                          2              ⁢                              N                ave                                                          =    0    ,         where Nave is the composite complex refractive index Nave=n+jk, Ni and f1 are the complex refractive index and volume fraction respectively for the i'th constituent, n is the conventional refractive index, j=√−1 , and k is the absorption coefficient.        
Conventionally, BEMA is applied to unordered mixtures. In the case of periodic structures, which are important here, an additional effect occurs namely form birefringence. This effect causes the system to be optically anisotropic even with isotropic constituents. Consequently, the optical properties of the system now have to be described by the so-called dielectric (3*3) tensor containing the ordinary N0 and extraordinary Neo complex refractive indicies. In order to apply the BEMA approach, an averaged N, namely Nave=1/3Neo1+1/3Neo2 is used resulting in a scalar volume fraction, f.
In accordance with the invention, a new optical method which we refer to as Spectroscopy Anisotropy Micro-Ellipsometery (SAME) is used for determination of the dielectric of the dielectric tensor N which then is converted into Nave and subsequently used in the BEMA-equation above. Both an apparatus (revolving ellipsometer) as well as methods to derive taper angle, τ, quantitatively or by calibration are used. The method according to the invention allows the measurement of an assembly of structures thus improving the statistical value of the measurement. The assembly-size can be adjusted as desired and depends on the spot size of the used optical apparatus only.
In accordance with one embodiment of the invention, a system is provided for measuring surface features having form birefringence. The system includes a radiation source for providing radiation having a selectable wavelength incident on a surface having surface features. A radiation detecting device is provided for measuring characteristics of the incident radiation after being reflected from the surface features, such measurement being made at each of a plurality of the selectable wavelengths. A rotating stage is included for rotating the surface such that incident radiation at each of the plurality of selectable wavelengths is directed at different angles due to the rotation of the rotating stage. A processor is provided for processing the measured characteristics of the reflected light for each of the plurality of selectable wavelengths and correlating the characteristics to measure the surface features.
In one embodiment, a method is provided for measuring feature sizes having form birefringence. The method includes providing a surface having surface features thereon; radiating the surface features with light having a first wavelength and a first polarization; measuring a reflected polarization of light having the first wavelength reflected from the surface features; rotating the surface features by rotating the surface to measure the reflected polarization of the reflected light having the first wavelength at least one new rotated position; radiating the surface features with light having a second wavelength and the first polarization; measuring a reflected polarization of light having the second wavelength reflected from the surface features; rotating the surface features by rotating the surface to measure the reflected polarization of the reflected light having the second wavelength at least one new rotated position; and correlating the reflected polarization from the light having the first and second polarizations to surface feature sizes.
In accordance with one embodiment of the invention, an apparatus is provided for measuring critical dimensions on an anisotropic sample at a high lateral resolution, such anisotropic sample showing form birefringence. The apparatus includes an ellipsometer for measuring ellipsometric parameters Δ and Ψ. The ellipsometer directs a linearly polarized incident light having a selectable wavelength onto a selected area of the sample to generate an elliptically polarized reflected light. The ellipsometer compares the linearly polarized incident light and the elliptically polarized reflected light at each of a plurality of the selectable wavelengths to measure the ellipsometric parameters Δ and Ψ. A rotating stage is rotatably disposed below the ellipsometer for rotating said sample so as to vary an angle of rotation α about a center of rotation axis, said center of rotation axis being aligned with the ellipsometer wherein said ellipsometer correlates said ellipsometric parameters Δ and Ψ to said angle of rotation α to determine the critical dimension at said selected area of the sample at a high lateral resolution.
As is also known in the art, it is important to determine rotational error, diffα, between two overlaying patterns.
In accordance with another feature of the invention, a method is provided for determine rotational error, diffα, between two overlaying patterns. The method includes, radiating the first pattern with light having a first polarization; measuring a reflected polarization of light having the first wavelength reflected from the surface features; determining a dielectric tensor from such measured reflected light; generate the second pattern over the first pattern; radiating the first pattern with the second pattern over the first pattern with light having the first polarization; measuring a reflected polarization of light from the first and overlaying second patterns; determining both the dielectric tensor and the Euler angles of the second pattern using the determined dielectric tensor of the first pattern to the determine rotational error, diffα.
In one embodiment the method includes: measuring Δ (α) and Ψ (α), curves of a first pattern showing birefringence; determine the dielectric tensor components of the first pattern by fitting the measured Δ (α) and Ψ (α) curves of this first pattern; generating the second pattern over the first pattern; measuring Δ (α) and Ψ (α) curves of this two overlaying patterns; determining both the dielectric tensor and the Euler angles of the second pattern using the aforementioned dielectric tensor of the first pattern as known parameters.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.